Electron-multiplier-ionizer mass spectrometer

ABSTRACT

An electron multiplier used together with additional elements to form a simple spectrometer for mass distribution studies. The multiplier is used to multiply a few electrons to provide an electron cascade at the multiplier output end which provides a trigger pulse, serving as a time reference and to ionize molecules at the output end. The multiplier serves as a path for positive ions which accelerate towards the multiplier&#39;&#39;s input end, whereat they collide with a surface to produce secondary electrons, which are then multiplied by the multiplier as they accelerate to the output end to provide output pulses, whose time displacements from the trigger are indicative of the mass to charge ratio of the created ions.

United States Patent Cone [151 3,663,810 [4 1 May 16,1972

[72] Inventor: Donald R. Cone, Palo Alto, Calif.

[73] Assignee: Stanford Research Institute, Menlo Park,

Calif.

[22] Filed: Feb. 14, 1969 [21] Appl.No.: 799,363

3,128,408 4/1964 Goodrich et al ..250/207 Primary Examiner-James W. Lawrence Assistant Examiner-A. L. Birch Attorney-Lindenberg, Freilich & Wasserman ABSTRACT An electron multiplier used together with additional elements to form a simple spectrometer for mass distribution studies. The multiplier is used to multiply a few electrons to provide an electron cascade at the multiplier output end which provides a trigger pulse, serving as a time reference and to ionize molecules at the output end. The multiplier serves as a path for positive ions which accelerate towards the multiplier's input end, whcreat they collide with a surface to produce secondary electrons, which are then multiplied by the multiplier as they accelerate to the output end to provide output pulses, whose time displacements from the trigger are indicative of the mass to charge ratio of the created ions.

7 Claims, 3 Drawing Figures SOURCE OF CHARGE!) ARTICLE$ ELECTRON-MULTIPLIER-IONIZER MASS SPECTROMETER BACKGROUND OF THE INVENTION 1 Field of the Invention This invention generally relates to electron multiplier devices and, more particularly, to a mass spectrometer utilizing a strip electron multiplier.

. 2. Description of the Prior Art Many instruments are known at present in which electrons are produced to provide an output which represents a phenomenon or information of interest. For example, secondary electrons are produced when reading out information which is stored in a storage tube. Since the number of electrons which is produced is quite small, electron multiplication is generally performed in order to produce a detectable output. Electron multiplication is produced by arrangements or devices, known as electron multipliers.

One type of electron multiplier, known as a continuous dynode strip, consists of a pair of elongated plates connected to a source of potentials so as to produce a voltage gradient between its two ends. One end serves as the multiplier's input end and the other as its output end. By introducing a charged particle such as an electron, proton or other energy packet at the input end of a suitably operated multiplier, a process is initiated whereby electrons repeatedly collide with the walls of the plates, producing ever increasing numbers of electrons which finally exit through the output end of the multiplier. A single electron at the input end may be multiplied to produce an output pulse of to 10" electrons within a time spread of a few nanoseconds. The electron density at the output end is sufficiently high to be easily detectable to produce a meaningful output signal. Herebefore such electron multipliers have been used nearly exclusively for electron multiplication only. It has been discovered that such multipliers may, together with additional elements, be used to perform other than strict electron multiplication functions. In particular it has been discovered that with minor additional elements, such a multiplier may serve as a simple reliable spectrometer for mass distribution analysis, or as apparatus for vacuum studies.

OBJECTS AND SUMMARY OF THE INVENTION It is a primaryobject of the present invention to provide an apparatus which incorporates an electron multiplier which performs unique functions.

Another object of the invention is to provide an apparatus in which an electron multiplier is operable to perform functions other than conventional electron multiplication.

A further object of the invention is to provide an electronmultiplier-incorporating system for mass distribution analysis.

Still a further object of this invention is to provide a new and reliable system, particularly adaptable for vacuum system analysis.

These and other objects of the invention are achieved by providing an apparatus in which an electron multiplier is used, together with properly operated electrodes, to provide an indication of mass distribution in an environment, such as a vacuum chamber in which the multiplier is located. Briefly, in one embodiment of the invention one or a few essentially concurrent trigger-pulse-forming electrons from a low intensity electron emitting source, located behind an apertured input electrode, enter the multiplier. The electrons cascade down the multiplier, as in a conventional multiplier, to provide a large number of electrons or an electron cascade at the output end where a conventional collector electrode is located, to produce an output trigger pulse.

The electrons exiting the multiplier, are attracted by an applied electric field to the collector electrode, to produce the trigger pulse, used for triggering purposes, as will be described hereafter. Some of these electrons ionize molecules of a gas or gasses which reside in the vicinity of the collector electrode in the vacuum chamber, in which the multiplier is located. The exponential character of the trigger electron cascade confines the ionization probability volume to the output end of the multiplier. Positive ions thus created at the multiplier's output end are accelerated by the multipliers voltage gradient towards the multiplier's input end. Because of their lower lateral velocity, and due to the multipliers dimension and voltage gradient, the positive ions generally do not strike the multipliers walls except near its input end or the apertured electrode thereat. When striking the apertured electrode or the walls at the input end, one or more secondary electrons are released and they initiate a secondary cascade of electrons through the multiplier, thus amplifying the signal to a detectable level at the collector electrode. The ion transit times from the output end to the input end of the multiplier are roportional to the square root of mass over charge mass charge).

-Since the electron cascade transit time is much less than transit time of any of the plus ions and in any event the electron cascade transit time may be deemed a constant detectable signal, pulses from different positive ions are displaced in time following the ionizing trigger pulse in proportion to their masses. These time displacements are easily viewable on an oscilloscope or recordable to provide an indication of the mass distribution in the apparatus chamber.

The novel features of the invention are set forth with particularity in the appended claims. The invention will best be understood from the following description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified diagram of the multiplier-spectrometer of the present invention;

FIG. 2 is a simplified spectrum diagram, useful in explaining the invention; and

FIG. 3 is another simplified diagram of the multiplier-spectrometer, in which various potential sources are diagrammed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As may be seen from FIG. 1, the multiplier-spectrometer of the present invention is assumed to be housed in a vacuum chamber 12 in which gaseous constituents, hereafter simply referred to as gasses whose mass distribution is to be analyzed are present. The gasses are designated in FIG. I by circles 14.

The teachings of the present invention may best be explained in conjunction with a specific application, for example, vacuum studies analysis. As is known, vacuum tightness properties of a chamber may be determined as a function of the leakage of an inert gas such as helium into the chamber. A helium leak checker is generally employed for such purposes. To serve as a helium leak checker the novel apparatus of the present invention is placed in vacuum chamber which is designated by numeral 12 in FIG. 1. Helium which may leak into the chamber through suspected leak points is represented by the small circles 14.

The apparatus of the present invention includes an electron multiplier 15, which is assumed to be of the continuous dynode strip type, and therefore may be referred to as the strip multiplier. The multiplier, however, is not intended to be limited to the strip type. Any type multiplier including a tubular or a channel multiplier may be employed as multiplier 15. The strip multiplier has an input end 16 and an output end I7, which are formed between the strip multipliers parallel, spaced apart plates 15a and 15b. As is appreciated by those familiar with the art of electron multipliers, the ends of the plates are connected to a source of potential so as to provide a potential gradient between the input and output ends. In order to simplify FIG. 1, all the connections between such a source, which would be located outside chamber 12 and the strip multiplier 15, as well as connections to other elements of the invention, are purposely deleted. As shown in FIG. 1 a collector electrode 21 is located adjacent output end 17, while an apertured input electrode 22 is located adjacent input end 16. The arrangement also includes a source of charged particles, which is designated by numeral 25, and shown aligned behind input electrode 22.

The function of source 25 is to inject a charged particle in the multiplier through its input end to create one or more free electrons which initiate or trigger the electron multiplication process or the electron cascade through the multiplier. Source 25 may comprise a beta electron source, a pulsed and/or deflected thermionic emitter or any like source to provide one or more electrons, represented in FIG. 1 by dashed line 23, and shown entering the strip multiplier 15, through the aperture 28 of input electrode 22.

Due to the potential gradient across the strip multiplier and the electron charge these electrons initiate the process of repeated electron collisions with the inner walls of the multiplier, each collision producing more electrons which cascade toward the output end 17. As is appreciated by those familiar with the art, at each collision some electrons are absorbed by the multipliers wall. However, the number of electrons which are released is significantly greater than the number absorbed, thereby accounting for the increased number of free electrons which travel towards the output end. Multiplication factors of to 10 are not uncommon. In FIG. 1 dashed line 26 represents the path of the electron cascade toward output end 17.

Most of the electrons exiting through output end 17 are attracted by the collector electrode 21 to form an electron trigger pulse, whose function will be described hereafter. Briefly stated, however, the trigger pulse is used as a time reference. Some of the electrons, which exit the multiplier ionize the gas molecules located thereat, to form positive ions. These positive ions, shown as small xs, are designated in FIG. 1 by numeral 30. The exponential character of the electron cascade which results in the electron density being highest at the multipliers output end 17 essentially restricts the ionization probability to the molecules at the output end.

Due to the voltage gradient across the multiplier, the positive ions are accelerated toward the multipliers input end. The path of the ions toward the input end is represented by dash-dot line 32. In accordance with the teachings of the present invention, the multipliers geometry, its potential gradient and the potential of the electrode 22 are chosen so that all the ions of interest accelerate towards the input end striking either the electrode 22 or the multipliers walls near the input end. The collisions between the positive ions and either electrode 22, or the walls about the input end 16, produce one or more free secondary electrons. These secondary electrons initiate the process of the formation of another electron cascade (similar to the trigger pulse cascade) as multiplied electrons accelerate back towards the collector electrode 21, as indicated by line 33. When the electrons finally strike the electrode 21 an output pulse is produced.

It should be appreciated that since ion transit time (from the output end 17 to the input end 16 or electrode 22 in the path indicated by 32) is proportional to the square root of the massto-charge ratio, if all the positive ions 30 are of the same element, e.g., helium, they all arrive at the electrode 22 at substantially the same time. Consequently, a single electrode cascade will be produced travelling towards the collector electrode 21 via path 33 to produce a single output pulse. However, if ions of different elements, namely different ions are produced, the ions arrival times and hence, the related output pulses are distributed in time in a manner re ated to the ion mass distribution. Thus, by detecting the time variations from a reference time, such as the one provided by the trigger pulse from collector electrode 21, at the instant of ionization the mass distribution in the chamber can be determined.

FIG. 2 to which reference is now made is a simple spectrum diagram of a plurality of pulses distributed in time. Therein, numeral 40 designates a trigger pulse produced at or very close to the instant of ionization assumed to be t while numerals 41-47 represent out pulses produced at times 1,4 respectively as a function of different ions of ascending Qs, where Q represents the mass-to-charge ratio.

From the foregoing it should thus be appreciated that in accordance with the teachings of the present invention an electron multiplier together with some additional elements such as the electrodes 21 and 22 and source 25 are used to form a simple multiplier-spectrometer. In response to a charged particle or particles from source 25, such as a beta electron source, which acts as a random trigger generator, the multiplier 15 multiplies formed electrons and provides a short path (26) therefor to produce a confined burst of electrons at the output end 17. Most of these electrons attracted by collector electrode 21 produce a trigger pulse (40 in FIG. 2) which is used as a time reference. The rest of the electrons ionize molecules at the output end and the multiplier and its potential gradient provide an ion transit path (32) along which the ions accelerate towards the input end and the input electrode 22. The latter (and/or the walls near the input end) serves as an ion-tosecondary-electron conversion surface. The resulting secondary electrons, which are created in a time sequence related to the ion transit times which depend on the ionized mass distribution at the output end, initiate the formation of electron cascades, or electron multiplication as they are accelerated towards the output end with multiple collisions to produce detectable output pulses at the collector electrode.

It should be appreciated that the trigger pulse and the one or more output pulses from the collector electrode may be recorded or viewed to provide an indication of the mass distribution. For example, the trigger pulse may be used to initiate a sweep, e.g., l ,usec. sweep, on an oscilloscope. Assuming that all ions of interest would result in the production of output pulses within l0s to l00s of nanoseconds after the trigger pulse, all the desired output pulses would appear on the oscilloscope during the l ,usec. sweep, at points therealong depending on the ion mass distribution. Their quantities will be portrayed by the repetitive appearance of pulses at time delays along the sweep corresponding to their respective transit time.

Altemately, time delay units and gates or coincidence circuits may be employed to provide outputs at selected time segments corresponding to particular charge/mass ratios, in order to determine the presence in the apparatus chamber of particular molecules. Such an arrangement may be used, for instance, to determine the presence of helium in the chamber in vacuum studies.

Reference is now made to FIG. 3 which is a simplified diagram of the multiplier 15, the electrodes 21 and 22, shown connected to three potential sources, such as batteries 51, 52 and 53. The function of battery 51 which is connected across the multiplier 15 is to provide the potential voltage thereacross, while the batteries 52 and 53 provide potential difference between the opposite ends of the multiplier and electrodes 21 and 22, respectively. It should be pointed out that in order to insure that any of the positive ions of interest strike only the electrode 22 or the multipliers wall very near the input end 16, it is necessary to establish a suitable L/D ratio for the multiplier, where L is its length and D is the distance between the plates 15a and 15b. It has been calculated that for an L/D ratio of 83, the desired effect is achieved. The potential of battery 51 is typically between 2-4 kV, e.g., 3 kV. The voltage provided by each of batteries 52 and 53 may be in the order to 20 V to V. As shown in FIG. 3, the collector electrode may be connected to a reference potential such as ground, through a resistor R and through a coupling capacitor C to an output stage 60. It is stage 60 which includes the circuits which utilize the trigger pulse and one or more of the output pulses supplied by collector electrode 21, which appear as voltage pulses across resistor R.

From the foregoing it should thus be appreciated that in accordance with the teachings of the present invention the strip multiplier 15 performs several functions. It is used, together with the associated circuitry, to first multiply the triggering electrons from source 25 to produce and sense its own trigger pulse. Then, it provides a field through which positive ions are fed back to the multipliers input end and input electrode 22, whereat electron conversion takes place. Finally, it is used to multiply these electrons to produce one or more amplified output pulses. All of these functions are performed with substantially no change in the operating potentials.

Although a particular embodiment of the invention has been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art and, consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.

What is claimed is:

l. A spectrometer comprising:

a substantially evacuated chamber;

an electron multiplier in said chamber, said multiplier defining an input end and an output end and a channel, formed by the multipliers inner walls, and extending from said input end to said output end;

first potential source means for applying a direct-current potential difference across said multiplier, the multiplier at its output end being at a positive potential of x volts with respect to the potential at the input end;

electron collecting means adjacent said output end for providing a pulse as a function of collected electrons; electron trigger means in said chamber near the multipliers input end for directing a charged particle to said multiplier to produce at least one electron in said channel at said input end, the potential difference across said multiplier accelerating said electron in said channel towards said output end to provide at said output end a plurality of electrons as a result of a succession of electron collisions with the inner walls of said multiplier, said electron collecting means collecting a portion of said electrons to provide a trigger pulse and the rest of said electrons ionizing molecules of at least one gas present at said output end to form positive ions, which as a function of the potential difference across said multiplier are attracted to the input end of said multiplier to produce at least one secondary electron upon colliding with an ion-electron converting surface thereat, the transit time of each positive ion from the output end of said multiplier to the input end being a function of the ions mass-to-charge ratio, said at least one secondary electron being accelerated towards said output end and multiplied by said multiplier to provide a plurality of electrons collected by said electron collecting means to form at least one output pulse at a time difference from said trigger pulse which is related to the mass-to-charge ratio of the ion producing said secondary electron,

2. The arrangement as recited in claim 1 wherein said first potential means provide a potential gradient across said multiplier, whereby positive ions, formed at the multiplier's output end, travel in the multipliers channel towards said input end without colliding with the multiplier except near said input end, the collision resulting in the formation of at least one electron which is accelerated towards said output end to produce a plurality of electrons as a result of successive electron collisions with said multiplier,

3. The arrangement as recited in claim 2 said apparatus further including electrode means disposed adjacent said input end for providing a collision surface for said positive ions accelerated towards said input end, said electrode means providing at least one electron as a result of the collision of the positive ions therewith.

4. The arrangement as recited in claim 1 wherein said multiplier is of length L, the dimension of said channel in a direction perpendicular to the multiplier's length is I), and said first potential means is a source of direct-current potential which is related to L/D, and said first potential means is connected to the multiplier at its output and input ends with the output end being at a potential of x volts above the potential at said input end.

5. The arrangement as recited in claim 4 wherein L/D is equal or greater than 83 and x is in the range of 3,000.

6. The arrangement as recited in claim 4 wherein said multiplier is of the dynode strip type comprising first and second plates separated by said distance D to define the channel therebetween.

7. The arrangement as recited in claim 4 said apparatus further including electrode means disposed adjacent said input end for providing a collision surface for said positive ions accelerated towards said input end, said electrode means providing at least one electron as a result of the collision of the positive ions therewith.

* t it 

1. A spectrometer comprising: a substantially evacuated chamber; an electron multiplier in said chamber, said multiplier defining an input end and an output end and a channel, formed by the multiplier''s inner walls, and extending from said input end to said output end; first potential source means for applying a direct-current potential difference across said multiplier, the multiplier at its output end being at a positive potential of x volts with respect to the potential at the input end; electron collecting means adjacent said output end for providing a pulse as a function of collected electrons; electron trigger means in said chamber near the multiplier''s input end for directing a charged particle to said multiplier to produce at least one electron in said channel at said input end, the potential difference across said multiplier accelerating said electron in said channel towards said output end to provide at said output end a plurality of electrons as a result of a succession of electron collisions with the inner walls of said multiplier, said electron collecting means collecting a portion of said electrons to pRovide a trigger pulse and the rest of said electrons ionizing molecules of at least one gas present at said output end to form positive ions, which as a function of the potential difference across said multiplier are attracted to the input end of said multiplier to produce at least one secondary electron upon colliding with an ion-electron converting surface thereat, the transit time of each positive ion from the output end of said multiplier to the input end being a function of the ion''s mass-to-charge ratio, said at least one secondary electron being accelerated towards said output end and multiplied by said multiplier to provide a plurality of electrons collected by said electron collecting means to form at least one output pulse at a time difference from said trigger pulse which is related to the mass-to-charge ratio of the ion producing said secondary electron.
 2. The arrangement as recited in claim 1 wherein said first potential means provide a potential gradient across said multiplier, whereby positive ions, formed at the multiplier''s output end, travel in the multiplier''s channel towards said input end without colliding with the multiplier except near said input end, the collision resulting in the formation of at least one electron which is accelerated towards said output end to produce a plurality of electrons as a result of successive electron collisions with said multiplier.
 3. The arrangement as recited in claim 2 said apparatus further including electrode means disposed adjacent said input end for providing a collision surface for said positive ions accelerated towards said input end, said electrode means providing at least one electron as a result of the collision of the positive ions therewith.
 4. The arrangement as recited in claim 1 wherein said multiplier is of length L, the dimension of said channel in a direction perpendicular to the multiplier''s length is D, and said first potential means is a source of direct-current potential which is related to L/D, and said first potential means is connected to the multiplier at its output and input ends with the output end being at a potential of x volts above the potential at said input end.
 5. The arrangement as recited in claim 4 wherein L/D is equal or greater than 83 and x is in the range of 3,000.
 6. The arrangement as recited in claim 4 wherein said multiplier is of the dynode strip type comprising first and second plates separated by said distance D to define the channel therebetween.
 7. The arrangement as recited in claim 4 said apparatus further including electrode means disposed adjacent said input end for providing a collision surface for said positive ions accelerated towards said input end, said electrode means providing at least one electron as a result of the collision of the positive ions therewith. 